The present invention relates to a pneumatic tire for vehicle wheels provided with a tread pattern particularly appropriate for running on snow-covered road surfaces, comprising at least two series of transverse grooves disposed on opposite sides relative to an equatorial plane of the tire and symmetrically converging at said equatorial plane. At least two circumferential grooves are symmetrically spaced apart from the equatorial plane of the tire and confine, together with the transverse grooves, one or more rows of center blocks disposed symmetrically to the equatorial plane and at least two rows of shoulder blocks located on respectively opposite sides relative to the center block rows. Thus, the invention concerns motor-vehicle pneumatic tires of the so-called "winter" type.
It is known that modern tires of the above type are required to possess not only excellent behavioral features with reference to traveling on a snow-covered road surface, but also good operating features such as rolling smoothness, ride behavior on a wet road surface and good mileage. The ride behavior on a dry road surface and the ride noiselessness too, although of less importance as compared with the previously mentioned features, help in increasing quality evaluation of a winter tire.
Said behavioral and operating features are achieved by forming appropriate circumferential and transverse grooves in the tread band, which grooves are suitably sized and oriented for formation of blocks usually aligned in rows disposed consecutively side by side and extending circumferentially around the tire.
In addition, an indispensable detail for improving the behavioral running features of a tire on a snow-covered road surface is the presence of appropriate "lamellae" or "fins" in the blocks, that is a plurality of narrow cuts or grooves having a small-sized zig-zag pattern in a circumferential direction, and principally oriented transversely of the rolling direction. The task of these cuts is efficiently collecting and retaining snow, since snow-on-snow friction is, as known, stronger than rubber-on-snow friction.
In more detail, a type of winter tire which has been recently commercialized by the assignee has a pair of circumferential straight-extending grooves in the tread, which grooves are disposed symmetrically on each axial side of the equatorial plane of the tire and spaced apart by a distance substantially corresponding to half the overall width of the tread band.
These circumferential grooves are combined with two series of transverse grooves each extending with a gradually increasing inclination from an outer edge of the tread band until close to the equatorial plane.
The transverse grooves of the two series symmetrically converge toward the equatorial plane of the tire according to a given preferential rolling direction and have their respective vertices consecutively alternated with respect to each other along the circumferential extension of the tread band at its equatorial plane.
In such a tread band, the configuration of the transverse grooves is adapted to promote good traction qualities, by virtue of the mutual convergence of the grooves in the rolling direction.
The circumferential straight-extending grooves, in turn, enable the water present in the ground-contacting area of the tire to be efficiently eliminated during running on a wet road surface, thereby preventing the well known and dangerous aquaplane effect.
The traction capability and, as a whole, the ride behavior on a snow-covered road surface are assisted by the presence of closely spaced fins on the blocks which are generated by the intersection between the longitudinal and transverse grooves and by appropriate straight extending hollows associated with the transverse grooves, in the space between the two circumferential grooves, each of said hollows extending in a circumferential direction over a short length starting from the respective transverse groove.
The Applicants' efforts have been directed towards discovering how said tire could be further improved, in particular regarding the behavioral qualities on a snow-covered road surface.